Visit Mozilla's extension room site at http://extensionroom.mozdev.org to learn all about extensions. This location is useful both for creating extensions and using them.
Firefox extensions can be quirky and temperamental. Often extensions can conflict or interfere with each other.
You should back up your profile(s) before installing extensions (refer to Chapter 4, "Managing Profiles," for tips on how to back up a profile).
Extensions are specific to a user profile. If you use more than one profile, you have to install the extension for each profile where it is to be used.
Extensions normally install automatically. However, some extensions seem to break this functionality, forcing you to install manually using a manual download and a drag and drop into the Extensions manager.
Mozilla's Firefox extensions pages list about 500 extensions. More are to be found on the Internet!